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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Propagation of sound waves through a linear shear layer - A closed form solution

Closed form solutions are presented for sound propagation from a line source in or near a shear layer. The analysis is exact for all frequencies and is developed assuming a linear velocity profile in the shear layer. This assumption allows the solution to be expressed in terms of parabolic cylinder functions. The solution is presented for a line monopole source first embedded in the uniform flow and then in the shear layer. Solutions are also discussed for certain types of dipole and quadrupole sources. Asymptotic expansions of the exact solutions for small and large values of Strouhal number give expressions which correspond to solutions previously obtained for these limiting cases.

Scott, J. N.↗
NASA NTRS1993

Control of pressure fluctuations in the reattachment region of a supersonic free shear layer

Measurements of wall pressure fluctuations were made as part of an ongoing program to investigate the control of a turbulent, reattaching shear layer at Mach 2.9. The flow was disturbed near separation by air injection normal to the plane of the shear layer. This perturbation was found to dramatically increase the intensity of pressure fluctuations in the vicinity of reattachment. A local increase in shear layer growth rate was also observed in the perturbed flow.

Poggie, J.↗
NASA NTRS1993

Turbulence measurement in a reacting and non-reacting shear layer at a high subsonic Mach number

The results of two component velocity and turbulence measurements are presented which were obtained on a planar reacting shear layer burning hydrogen. Quantitative LDV and temperature measurements are presented with and without chemical reaction within the shear layer at a velocity ratio of 0.34 and a high speed Mach number of 0.7. The comparison showed that the reacting shear layer grew faster than that without reaction. Using a reduced width coordinate, the reacting and non-reacting profiles were very similar. The peak turbulence for both cases was 20 percent.

C. T. Chang↗
NASA NTRS1981

On the structure of turbulent wakes and merging shear layers of multielement airfoils

The structure of the attached turbulent flow in the vicinity of a NACA 4412 airfoil equipped with a single-slotted flap was studied. The airfoil/flap configuration was tested at a Mach number of 0.06 and a Reynolds number of 1.3 x 10 to the 6th in a 7- by 10-Foot Wind Tunnel. Surface-pressure measurements were made on the main airfoil and on the flap. Detailed measurements, obtained using a high-spatial-resolution laser Doppler anemometer, were made of the mean velocity flow field and of the second-order statistical quantities (Reynolds stresses) in the boundary layers, wakes, and merging shear layers. The experimental observations are compared with theoretical predictions of pressure, mean velocity, and Reynolds stress.

Olson, L. E.↗
NASA NTRS1990

Resonant instability of supersonic shear layers

A computer simulation of possible resonant instability of a supersonic shear layer is carried out. The resonance of two acoustic duct modes of the flow induced by periodic Mach waves generated by a wavy wall is sought. Results of the simulations are reported. Simulations are unable to document a resonant instability and the mixing characteristics remain unchanged. Possible weakness of the present simulations are discussed. A second set of simulations involving a mixing layer separating a supersonic and a subsonic stream were performed. A wavy wall placed adjacent to the supersonic stream to produce a set of periodic Mach waves terminating at the shear layer is modelled. The entire flow field is similar to that of an imperfectly expanded supersonic jet discharging into a subsonic coflowing stream for which enhanced mixing due to the onset of screech (feedback instability) is known to occur. The purpose of these simulations is to see if enhanced mixing and feedback instability would, indeed, take place. Some evidence of feedback oscillations is found in the simulated flow.

Tam, C. K. W.↗
NASA NTRS2006

CDUCT-LaRC Status - Shear Layer Refraction and Noise Radiation

A proposed boundary condition accounting for shear layer effects within the Ffowcs Williams-Hawkings radiation module of the CDUCT-LaRC code is investigated. The development and numerical justification of the boundary condition formulation are reviewed. An initial assessment of the effectiveness of the shear layer correction is conducted through comparison with experimental data. Preliminary results indicate that the correction provides physically meaningful modifications of the baseline predicted directivity patterns. Trends of peak directivity steepening and shifting that appeared in predicted patterns were found to follow similar structures in measured data, particularly at higher radiation angles.

Nark, Douglas M.↗
NASA NTRS1990

Effects of compressibility on the characteristics of free shear layers

A high Reynolds number two-dimensional constant pressure compressible shear layer was formed at the trailing edge of an 0.5 mm-thick splitter plate. Convective Mach numbers of 0.51 and 0.64 were investigated using a two-component coincident LDV for the measurements. For the lower convective Mach number case, the nondimensionalized shear-layer and vorticity thickness growth rates were over 20 percent higher and the momentum thickness growth rate was over 30 percent higher than those of the higher convective Mach number case. The results seen to indicate that both small scale and large scale mixing are reduced with increasing convective Mach number.

Samimy, M.↗
NASA NTRS1977

Variable thickness shear layer aerodynamics revisited

The constant boundary-layer thickness (BLT) figuring in the Ventres (1975) Kernel function can be replaced by a slowly varying BLT, in the case of shear layers of slowly varying thickness. A simplification of the extension by Chi (1976) of Ventres' solution is put forward. The Kernel function in this instance relates pressure on the lifting surface to downwash over the surface. The results should also apply, formally, to three-dimensional compressible unsteady flows, but the accuracy in assuming slowly varying shear BLT remains to be determined. All variants of the shear layer model fail when the shear layer thickness varies rapidly.

Dowell, E. H.↗
NASA NTRS1993

Comparison of the initial development of shear layers in two-dimensional and axisymmetric ejector configurations

A two-phase experimental investigation designed to study the development of shear layers in axisymmetric and two-dimensional single-nozzle ejectors has been completed. In this study, combinations of similar and dissimilar gases were used as the supersonic primary and subsonic secondary. Test cases included combinations of air/air, argon/air and helium/air as the supersonic primary and subsonic secondary, respectively. Similar flow conditions were studied for each ejector configuration. Mixing of the gases occurred in a constant-area tube, where the inlet pressure was maintained at 34.5 kPa. The cases studied resulted in convective Mach numbers that range between 0.06 and 1.9. The data gathered shows differences between the initial shear-layer development for the two ejector geometries, and also between the different test cases studied for each ejector configuration. The measured growth rates for the axisymmetric ejector are more than twice those measured for the two-dimensional ejector. However, in both cases the results show that compressibility has a reducing effect on the growth rate. Further, in the region immediately after the inlet to the mixing tube, compressibility seems to affect the ejector shear layers in a manner similar to that of two-stream two-dimensional mixing layers.

Dufflocq, M.↗
NASA NTRS1994

Turbulent Heat Transfer Characteristics in the Shear Layer of a Separated Flow

Experiments were performed to study the evolution of the heat transfer structure in a separated free shear layer region of an incompressible separated turbulent boundary layer flow behind a backward-facing step. While there is an abundance of velocity field measurements of separated flows, heat transfer measurements are rather scarce, thus limiting assessment of the heat transfer physics and its accurate modeling. The purpose of the paper is twofold: to improve an understanding of effects of flow separation on heat transfer characteristics, and to provide data for turbulence modeling and computation. The boundary layer upstream of the step was turbulent and fully developed. A constant temperature surface boundary condition was imposed upstream and downstream of the step for the heat transfer study. An internal mixing-layer like flow forms and grows from the step lip within the original boundary layer. The turbulent structure of the flow evolving downstream, however, does not switch immediately to that of a mixing layer over the entire shear layer thickness. Measurements of mean and fluctuating velocity and temperature fields indicate that the internal layer spreads gradually in the transverse direction while the outer part of the original boundary layer is effectively unperturbed. The results in this paper have not been previously reported.

Jovic, S.↗
NASA NTRS2001

Time-Accurate Simulations and Acoustic Analysis of Slat Free-Shear Layer

A detailed computational aeroacoustic analysis of a high-lift flow field is performed. Time-accurate Reynolds Averaged Navier-Stokes (RANS) computations simulate the free shear layer that originates from the slat cusp. Both unforced and forced cases are studied. Preliminary results show that the shear layer is a good amplifier of disturbances in the low to mid-frequency range. The Ffowcs-Williams and Hawkings equation is solved to determine the acoustic field using the unsteady flow data from the RANS calculations. The noise radiated from the excited shear layer has a spectral shape qualitatively similar to that obtained from measurements in a corresponding experimental study of the high-lift system.

Khorrami, Mehdi R.↗
NASA NTRS2002

Time-Accurate Simulations and Acoustic Analysis of Slat Free-Shear-Layer

Unsteady computational simulations of a multi-element, high-lift configuration are performed. Emphasis is placed on accurate spatiotemporal resolution of the free shear layer in the slat-cove region. The excessive dissipative effects of the turbulence model, so prevalent in previous simulations, are circumvented by switching off the turbulence-production term in the slat cove region. The justifications and physical arguments for taking such a step are explained in detail. The removal of this excess damping allows the shear layer to amplify large-scale structures, to achieve a proper non-linear saturation state, and to permit vortex merging. The large-scale disturbances are self-excited, and unlike our prior fully turbulent simulations, no external forcing of the shear layer is required. To obtain the farfield acoustics, the Ffowcs Williams and Hawkings equation is evaluated numerically using the simulated time-accurate flow data. The present comparison between the computed and measured farfield acoustic spectra shows much better agreement for the amplitude and frequency content than past calculations. The effect of the angle-of-attack on the slat's flow features radiated acoustic field are also simulated presented.

Khorrami, Mehdi R.↗
NASA NTRS1990

Non-linear coherent mode interactions and the control of shear layers

A nonlinear integral formulation, based on local linear stability considerations, is used to study the collective interactions between discrete wave-modes associated with large-scale structures and the mean flow in a developing shear layer. Aspects of shear layer control are examined in light of the sensitivity of these interactions to the initial frequency parameter, modal energy contents and modal phases. Manipulation of the large-scale structure is argued to be an effective means of controlling the flow, including the small-scale turbulence dominated region far downstream. Cases of fundamental, 1st and 2nd subharmonic forcing are discussed in conjunction with relevant experiments.

Nikitopoulos, D. E.↗
NASA NTRS1992

Vortex-induced disturbance field in a compressible shear layer

The disturbance field induced by a small isolated vortex in a compressible shear layer is studied using direct simulation in a convected frame. The convective Mach number, M(sub c), is varied from 0.1 to 1.25. The vorticity perturbation is rapidly sheared by the mean velocity gradient. The resulting disturbance pressure field is observed to decrease both in magnitude and extent with increasing M(sub c), becoming a narrow transverse zone for M(sub c) greater than 0.8. A similar trend is seen for the perturbation velocity magnitude and for the Reynolds shear stress. By varying the vortex size, we verified that the decrease in perturbation levels is due to the mean-flow Mach number and not the Mach number across the vortex. At high M(sub c), the vortex still communicates with the edges of the shear layer, although communication in the mean-flow direction is strongly inhibited. The growth rate of perturbation kinetic energy declines with M(sub c) primarily due to the reduction in shear stress. For M(sub c) greater than or equal to 0.6, the pressure dilatation also contributes to the decrease of growth rates. Calculation of the perturbation field induced by a vortex doublet revealed the same trends as in the single-vortex case, illustrating the insensitivity of the Mach-number effect to the specific form of initial conditions.

Papamoschou, D.↗
NASA NTRS1996

Effect of a delta tab on fine scale mixing in a turbulent two-stream shear layer

The fine scale mixing produced by a delta tab in a shear layer has been studied experimentally. The tab was placed at the trailing edge of a splitter plate which produced a turbulent two-stream mixing layer. The tab apex tilted downstream and into the high speed stream. Hot-wire measurements in the 3-D space behind the tab detailed the three velocity components as well as the small scale population distributions. These small scale eddies, which represent the peak in the dissipation spectrum, were identified and counted using the Peak-Valley-Counting technique. It was found that the small scale populations were greater in the shear region behind the tab, with the greatest increase occurring where the shear layer underwent a sharp turn. This location was near, but not coincident, with the core of the streamwise vortex, and away from the region exhibiting maximum turbulence intensity. Moreover, the tab increased the most probably frequency and strain rate of the small scales. It made the small scales smaller and more energetic.

Foss, J. K.↗
NASA NTRS1983

Reattaching turbulent shear layers with perturbed structure

The reattachment of a fully turbulent, two-dimensional shear layer downstream of a backward-facing step has been studied experimentally. The work examines the effect of modifications in shear layer structure upstream of reattachment. A series of experiments were conducted in a low-speed wind tunnel using specialized instrumentation suited to the highly turbulent, reversing flow near reattachment. Accurate characterization of the time-mean features of the reattaching flows was possible. Assuming linear scaling normalized on distance from reattachment, distributions of the normalized pressure coefficients, the time-averaged skin friction coefficient, and the forward flow fraction appear universal for two-dimensional reattachment, independent of initial conditions and steep height, for given duct geometry (area ratio) and for high step height Reynolds numbers. The results suggest universal flow structure in the reattachment zone.

Westphal, R. V.↗
NASA NTRS1988

Nonlinear roll-up of externally excited free shear layers

The effects of strong critical-layer nonlinearity on the spatially growing instabilities of a shear layer between two parallel streams are considered. A composite expansion technique is used to obtain a single formula that accounts for both shear-layer spreading and nonlinear critical-layer effects. Nonlinearity causes the instability to saturate well upstream of the linear neutral stability point. It also produces vorticity roll-up that cannot be predicted by linear theory.

Goldstein, M. E.↗